1. Field of the Invention
The present invention relates to a compact and inexpensive zoom lens.
2. Description of the Related Art
Digital still cameras or video cameras having a solid-state imaging device such as a charge coupled device (CCD) or a complementary metal-oxide semiconductor (CMOS) are widely used nowadays. In this regard, the demand for megapixel camera modules is increasing. Inexpensive digital cameras have a resolution of five megapixels or greater and provide high-definition performance. Reduction of size, weight, and cost is desired in optical imaging devices including mobile phone cameras or digital cameras using an imaging device such as a CCD or a CMOS.
In order to satisfy demands for size reduction, a retractable lens barrel in which a lens is extended to a certain position in a photographing mode and the lens is retracted into a camera body in a non-photographing mode is widely used. A camera having such a retractable lens barrel may have a small thickness and improved portability only if the distance between lens groups is minimized when the lens barrel is retracted into the camera body. In order to make the retractable lens barrel small and thin, the number of lens groups should be reduced. At the same time, high optical performance has to be provided for a large number of pixels.
In order to satisfy the above-described demand, a zoom lens including three lens groups is widely used. An example of a zoom lens system having a variable magnification of two times or higher and including a compact photographing lens system, is a well-known zoom lens system which includes, in order from object side to image side, a first lens group having a negative refractive power, a second lens group having an overall positive refractive power, and a third lens group having a positive refractive power and varies magnification by varying a distance between lens groups.
Another example is a zoom lens including two lens groups such as a first lens group having a negative refractive power and a second lens group having a positive refractive power. The focal length of the zoom lens system including the two lens groups is determined by multiplying a magnification of the first lens group by the magnification of the second lens group. During zooming from wide to telephoto the distance between the first and second lens groups varies and thus the focal length also varies. Since the magnification power of the back lens group is relatively small, an aberration variation in accordance with a varied magnification is small but the aperture of the front lens group is relatively large and the length of the retro-focus type system is relatively long. Thus, reduction of an optical full length is restrictive and magnification is restricted to approximately two times. However, despite the high magnification achieved, reduction of size and cost may not be easily achieved concurrently.